In many very large scale integrated semiconductor (VLSI) devices, gate electrodes typically include a doped silicon film and a refractory metal silicide film. In some of these devices, the refractory metal silicide is tungsten silicide, which is typically formed by reacting tungsten hexafluoride (WF.sub.6) with silane (SiH.sub.4). One of the problems with tungsten hexafluoride is that the fluorine can cause gate dielectric thickening. The thickening changes the electrical characteristics of the gate dielectric and is undesired. After forming the tungsten silicide film, an anti-reflective coating (ARC) is formed over the tungsten silicide. Typically, this film includes silicon nitride or the like, but silicon nitride adheres poorly to tungsten silicide. Therefore, a thin undoped amorphous silicon film is used between the tungsten silicide and silicon nitride to promote adhesion.
Another problem with the prior art is typically two doping steps are used in forming the doped silicon film. More specifically, N+ doped silicon is used over n-channel transistors and P+ silicon is used over the p-channel transistors. A further problem with the prior art gate electrodes is that boron from the gate electrode can penetrate the substrate and change the threshold voltage of the p-channel transistors. This problem is more pronounced with a thin gate dielectric layer.
A need exists to form a gate electrode that has a work function near the middle of the band gap for the material of the substrate, does not cause the fluorine-related problems of gate thickening, and is resistant to boron penetration. A need also exists for forming the gate electrodes without using a complicated process or by having to use marginal processing steps.